Rich,

I missread what you originally said

However, I have yet to find a mechanical gauge under £200 that
has any sort of reasonable accuracy.

However....

2 bar = (2 x 14.5 psi) = 29.0 psi
(rounded down to one decimal place before anyone moans)

29.0 - 26.4 = 2.6 psi

If you allow 1% for monitor accuaracy (0.9 psi for a 2 bar scale/3 bar map)
That means the gauge at best is 1% in-accurate.

(Assuming all is functioning correctly)

Also, dont forget,, the viewing angle of the gauge will affect how you
read the scale.